1. Field of the Invention
The present invention is directed to communication systems. More particularly, the invention is directed to receivers in wireless communication systems. Even more particularly, the present invention is directed to noise floor calibration and signal strength detection systems for such wireless communication system receivers.
2Background
The use of receivers in wireless systems such as radio and cellular communication systems is well-known in the art. FIG. 1 shows a typical superheterodyne receiver design 10. Here, a radio frequency (RF) signal is received on antenna 15 and provided to RF amplifier 20. The RF signal is amplified by the RF amplifier 20 and in mixer 25 mixed with a signal from a local oscillator 30. This produces an intermediate frequency (IF) signal that is amplified in an IF amplifier 35 and filtered in a bandpass filter 40. The filtered IF signal is again amplified by an IF amplifier 45 and mixed in a product detector 50 with a signal from a beat frequency oscillator 55. The result is a signal that is amplified by a baseband amplifier 60 and digitized for further processing by an analog-to-digital (A/D) converter 65.
The physical layers of most radio systems are able to return a Received Signal Strength Indication (RSSI). This RSSI value is normally expressed in absolute signal power. However, for many purposes a relative signal strength between two measurements can serve as well.
The RSSI value is commonly used for the following purposes:
Clear Channel Assessment: Many protocols employ a Carrier Sense Multiple Access (CSMA) method for sharing the channel. This type of listen-before-talk protocol typically will have a threshold, expressed in absolute signal strength (dBm), at which the channel should be considered busy and transmissions should be held off. For example, the 802.11a wireless LAN protocol states that the receiver must detect any in-band signal above −62 dBm and declare the medium busy for the duration of this signal.
Feedback to the user: An indication of the signal to noise ratio (SNR) of the received packet can be given to the user. This indication can be a relative measurement comparing the observed signal strength of the packet with the observed signal strength of the noise. The user can also be shown the absolute signal size of received packets. Both measurements can be used together to distinguish between poor reception due to a weak signal, and poor reception due to excessive noise.
Channel selection: The Media Access Controller (MAC) can use the received SNR sampled on several different channels to select the best frequency channel and access point to associate with. In this case the radio would be looking for both the amount of noise in a given channel as well as the strength and prevalence of interfering signals.
Roaming/Hand-off: Similar to channel selection, a station that is already associated with one access point (AP) can look for other APs that provide stronger signals. In this case, relative RSSI measurements can serve the requirements adequately. However, an absolute RSSI value may be more convenient, as a station may not want to even begin to search for another AP until the signals from its current AP fall below a certain level.
As this list indicates, both relative and absolute measurements of the received signals and the noise floor are useful in a radio system.
Discussion of the Available Art
A number of methods have been used to provide accurate receive signal strength indications; most of these are based on building circuits that themselves provide very high accuracy. However, each of the available approaches requires complicated designs, or the use of semiconductor technology with a high degree of accuracy and repeatability, leading to high costs.
The traditional method for accurately finding RSSI is illustrated in FIG. 9. In this case, the signal is received through the entire receive chain. Significant amounts of gain are added along the path so that the desired signal is large when its size is measured. The size at the antenna is then predicted by subtracting the receive gain from the measured size of the signal in the baseband.
An advantage to this approach is that the received signal can be put through a narrow baseband filter, which removes any unwanted out of channel signals so that the power measurement can be based on only the desired signal.
However, there are a number of disadvantages to this approach. Traditional analog signal strength measurement circuits are tricky to design and require a high degree of analog accuracy and repeatability. While it is reasonable to achieve accuracy in bipolar integrated circuits, it is much more difficult in inexpensive CMOS technology. These inexpensive CMOS process fabrication technologies were designed for digital circuits, and do not commonly provide the accuracy or manufacture quality control and repeatability that makes design of accurate circuits convenient.
Even using bipolar technology, most signal strength indicators built into the analog domain need to be temperature compensated. This can be done with additional analog circuitry or by measuring the temperature and correcting the RSSI value digitally. In addition, a separate analog to digital converter (ADC) is required to digitize the RSSI signal, and possibly the temperature detector as well in order to temperature compensate the RSSI signal.
Along with the difficulty of making an accurate signal power measurement at baseband, there are difficulties in referring the measurement to the antenna. In order to estimate how big the signal was at the antenna, the gain between where signal strength is measured and the antenna must be known. This gain changes over temperature and process variations, especially in an inexpensive CMOS technology. In addition, variation in the components used to match the RF circuits can cause significant variation in gain from one unit to another.
Some of the variation in gain can be avoided by placing the signal magnitude detection circuit closer to the antenna. However, because the received signal can be very small, such a circuit is difficult to construct accurately. In addition, since it is measured at a point before the narrow channel filter, the measured signal size will depend not only on the desired signal, but will also include any undesired signals in nearby channels. This limited filtering effect can be a problem even with baseband signal detection in modern receivers, as much of the receive signal filtering is done in the digital baseband, after analog RSSI detection.